This invention generally concerns Raman optical amplifiers, and is particularly directed dispersion managed discrete Raman amplifiers.
Optical amplifiers for amplifying photonic signals transmitted through optical fiber networks are well known in the art. Such amplifiers are used to extend transmission distances and to compensate for losses from various network elements. Presently, there are several known types of optical amplifiers, including erbium-doped fiber amplifiers (EDFAs), and Raman amplifiers.
EDFAs typically comprise at least one pump laser whose output is optically coupled to at least one coil of erbium-doped optical fiber. In operation, the output of the pump laser excites the atoms of the erbium-dopant within the fiber. These excited atoms release their excess energy in proportion to the strength of the incoming optical signal, which results in an amplified output. By contrast, Raman amplifiers achieve amplification without the need for erbium-doped optical fibers. Therefore, Raman amplifiers may use optical fibers without Er dopant as their gain fiber.
In one type of Raman amplifier, the output of a pair of orthogonally polarized pump-diode lasers provides backward propagating pump power in the gain fiber. Alternatively, a single pump and a de-polarizer may also be utilized to provide pump power in the gain fiber. Forward-propagating signals are amplified in the gain fiber by higher energy (shorter wavelength) pump photons scattering off the vibrational modes of the optical fiber""s lattice matrix and coherently adding to the lower-energy (longer wavelength) signal photons.
Raman amplifiers may be one of two types, depending upon the type of the gain fiber used therein. Distributed Raman amplifiers advantageously use the silica based optical transmission fiber itself as the gain fiber. By contrast, discrete Raman amplifiers typically utilize their own silica based optical fiber as the gain fiber. While the dopant used in the gain fiber of a discrete Raman amplifier is typically the same as used in the optical transmission fiber (e.g., germanium), the gain fiber of the discrete Raman amplifier usually contains higher concentrations of dopant (such as germanium) than a conventional optical transmission fiber and is designed to operate with a decreased fiber effective area.
The Raman gain efficiency of an optical fiber as a Raman amplifier is characterized by a figure of merit defined as   F  =            G      R              A      ⁢              xe2x80x83            ⁢              α        p            
where GR is the Raman scattering coefficient of the optical fiber material at the specified pump wavelength, A is the effective area of the optical fiber and xcex1p is the attenuation of the optical fiber at the pump wavelength.
Silica based Ge-doped optical fiber manufactured for use in discrete Raman amplifiers has a small effective area A, typically about 10-20 xcexcm2. The addition of germanium to the silica based optical fiber increases the Raman scattering coefficient, GR. Because of this, the figure of merit F of this optical fiber is typically greater than 20 for pump wavelengths in the 14xc3x97xc3x97 nm band. In comparison the figure of merit for a typical optical transmission fiber is about 7 or 8.
High levels of germanium dopant and small effective areas of the fibers utilized in discrete Raman amplifiers have an undesirable side-effect in that the nonlinear refractive index n2/A of these fibers is very high compared to that of transmission fibers. The net refractive index is defined by the equation n=n0+n2P/A, where n0 is the linear part of the net refractive index (which is independent of the optical power P propagating through the fiber) and n2/A is the nonlinear part (which is dependent on the optical power P propagating through the fiber). For example, n2/A in a Discrete Raman gain fiber is about 3xc3x9710xe2x88x929 Wxe2x88x921, compared to about 0.3xc3x9710xe2x88x929 Wxe2x88x921 in a transmission fiber, such as for example, SMF-28(trademark) available from Corning, Inc. of Corning, N.Y. The large nonlinear refractive index can lead to unwanted nonlinear interactions such as self-phase modulation (SPM), cross-phase-modulation (XPM) and four-wave mixing (FWM). In addition, this type of highly doped and small effective area fiber typically has dispersion in the range xe2x88x9220 to xe2x88x9230 ps/nm/km. Typical optical transmission fibers have dispersion values of +5 to +19 ps/nm/km.
System penalties from FWM can be significantly reduced or eliminated by using gain fibers which have a high dispersion (greater than |20| ps/mn/km) to prevent phase-matching between different signal channels. However, the gain fiber utilized in discrete Raman amplifiers is generally several kilometers in length and using gain fibers which are highly dispersive fibers will result in the discrete Raman amplifier with a significant net negative dispersion.
A highly dispersive discrete Raman amplifier will be a disadvantage in systems employing dispersion managed cable or dispersion managed fiber where either no dispersion or only a limited amount of dispersion is required at site of the amplifier.
According to present invention an optical amplifier comprises: (i) an input port for providing optical signal to the amplifier; (ii) an output port for providing amplified optical signal out of the amplifier; (iii) at least two optical fibers, one optical fiber having positive dispersion D1 of greater than 10 ps/nm/km in a 1550 nm to 1620 nm wavelength range, the other fiber having negative dispersion D2 of less than xe2x88x925 ps/nm/km in a 1550 nm to 1620 nm wavelength range, wherein the length of each of said optical fiber is chosen to provide the amplifier with a predetermined amount of dispersion.
According to one embodiment of the present invention a discrete Raman amplifier comprises: (i) an input port providing optical signal to said amplifier; (ii) an output port providing amplified optical signal out of the amplifier; (iii) at least two optical fibers, one optical fiber having positive dispersion D1 of greater than 10 ps/nm/km in a 1550 nm to 1620 nm wavelength range, and length of 5 m to 2.5 km, the other fiber having a negative dispersion D2 of less than xe2x88x925 ps/nm/km in a 1550 nm to 1620 nm wavelength range and length of 5 m to 2.5 km, wherein the length of each of the two optical fibers is chosen to provide this amplifier with substantially zero dispersion over the signal range, such that the dispersion of the amplifier is less than 25 ps/nm over the signal wavelength range.